The present invention relates to a lathe for processing a workpiece into a nonaxisymmetric shape such as a cam shape or the like.
A conventional NC (numerically controlled) cam cutting lathe includes a head stock for holding a workpiece and a tool rest for holding a turning tool or bit for cutting the workpiece. Usually, the workpiece has a round bar shape before cutting and one end of this round bar is secured to a spindle mounted on the head stock. On cutting, the spindle is rotated around an axis of the workpiece and the tool rest is slidably moved in the direction of the axis of the workpiece and in the direction perpendicular to the moving direction of the head stock. The knife edge of the bit held on the tool rest is contacted with the side surface of the workpiece so as to cut it. As the tool rest is moved in the moving direction, the contact point of the bit with respect to the workpiece is successively axially move to enable a cutting of the workpiece in a predetermined area in the axis direction of the workpiece. Also, by moving the tool rest towards the workpiece, the contact point of the bit with the workpiece is successively moved in the cutting direction deeper into the interior to carry out deeper cutting.
The above-described lathe structure is the same as a usual lathe. However, in this lathe, the distance from the rotary center to the workpiece surface in the workpiece is not fixed in the radial direction, and thus it is required to move the bit forwards or backwards in the cutting direction in correspondence to the cam shape in synchronism with the rotation of the workpiece. That is, in the lathe for cutting the cam, a driving mechanism or a feeding mechanism for moving the bit forwards or backwards in synchronism with the rotation period of the workpiece is required. In particular, with the increase of the rotation speed of the workpiece for reducing the machining time, the moving of the tool at a quick speed is required.
Accordingly, it is necessary to move the heavy tool rest at high speed, and the tool rest and its sliding saddle surface of a bed are worn. Also, a screw surface of a feeding screw of the feeding mechanism is partially worn at only a frequently used part.
Further, when the tool rest is changed from the rearward movement to the frontward movement and vice versa, the moving direction of the tool rest is suddenly changed and a large shock is given to the feeding mechanism of the tool rest. This large shock often breaks parts of the feeding mechanism or at least deteriorates the accuracy of the same.
In the conventional lathe, in order to avoid the above-described problems, the cutting speed, that is, the workpiece rotation speed, is required to be low.
Also, since the bit is secured to the tool rest, the rotation axis of the workpiece, that is, the angle with respect to the spindle, is fixed. However, regarding the workpiece, the distance (radius) from the rotation axis to the workpiece surface is not equal in the radial direction. Hence, in the cam cutting, the contact angle of the bit with the cam surface is not fixed and is always varied. Particularly, at the large cam radius change part, the bit contact angle is substantially changed. Thus, with the change of the contact angle of the bit, the angle (effective rake angle) of the cutting face of the bit with respect to the cutting surface of the workpiece is changed. This effective rake angle gives a large influence to a cutting force at the cutting time and, with the change of the effective rake angle, the cutting force is changed.
That is, as shown in FIG. 14, when a workpiece 110 is cut into a nonaxisymmetric shape, by only a frontward or rearward movement, that is, an X-direction movement of a tool or cutter 114, a contact angle (rake angle) a of the cutter 114 with respect to the workpiece 110 is always varied depending on the cutting position. When this rake angle a is changed, a shear angle b of a tip 113 is changed. A size of a sliding surface 113a of the tip 113 is changed and thus the cutting force is changed. As described above, changing the effective rake angle to shift out of a design value creates a bad influence on the smoothness of the cutting surface, and the smoothness required to a finish surface can not be satisfied.
Further, in order to avoid a contact of the rear surface of the bit with the cutting surface, as shown in FIG. 13, it is necessary to also increase a clearance angle c of a bit 114. Hence, an included angle e becomes small and a knife strength is drastically reduced.